How Testing Shows the Weaknesses of the DBMS

We'll talk not about the testing intself, but about databases. It is clear that a company's modern IT infrastructure can dynamically scale and distribute the user load. But there is always a weak link, and these are relational databases - an integral attribute of all modern products. How to understand that the application can still scale, but the database does not? What to do in this case? How can this problem be solved in a simple and non-trivial way? How can you collect performance metrics and improve application performance?